Probing Carrier Depletions on Grain Boundaries in Polycrystalline Si Thin Films by Scanning Capacitance Microscopy
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1066-A04-02
Probing Carrier Depletions on Grain Boundaries in Polycrystalline Si Thin Films by Scanning Capacitance Microscopy C.-S. Jiang, H.R. Moutinho, B. To, P. Dippo, M.J. Romero, and M.M. Al-Jassim National Renewable Energy Laboratory, Golden, CO, 80401
ABSTRACT Grain boundaries (GBs) in polycrystalline Si thin-film solar cells are believed to limit the photovoltaic efficiencies. In this paper, we report on a nanometer-resolution measurement of the carrier depletion at the GBs, using scanning capacitance microscopy (SCM). The SCM images exhibit the following features: (1) Carrier concentrations are lower at locations around the GBs than on center regions of the grains; (2) The depletion width at the GBs varies considerably, between 0 and 100 nm, depending on individual GBs; (3) Intra-grain carrier depletion was also observed at point and line defects; and (4) The faceted features that were observed on the topography of the as-grown film surface appeared on the SCM images even after the film surface was polished flat. The direct measurement of the carrier depletion on the GBs demonstrates that the GBs in Si thin films indeed create charged gap states. The nonuniformity of the carrier depletions suggests that the gap states depend on specific GB structures, which should relate directly to the grain orientations and grain facets adjacent to the GB. The depletion around the intragrain defects indicates that the defects are charged and can be recombination centers, and thus, harmful to device performance. This paper reports the first step of our studies toward understanding the relationships between the electronic and structural properties on specific GBs. INTRODUCTION Polycrystalline Si thin film is a promising candidate in next-generation thin-film solar cells because of its advantages in material saving, low cost, and potential high performance. In recent years, great efforts have been made to improve the quality of this material, and energy conversion efficiency of the devices steadily improved [1,2]. Enlargement of grain size and passivation of grain boundaries (GBs) in the polycrystalline material are considered effective ways to stimulate research and further improve efficiency [2–5]. Most of the literature reports that GBs in Si films harm the efficiency because GBs act as recombination centers for photoexcited carriers, and the carrier transports are scattered at the GBs by electrostatic potential barriers [2–4,6–8]. The GBs negatively impact all device performance parameters, which include short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) [3,6–8]. On the other hand, positive impacts of GBs were proposed by two-dimensional device simulation and by quantum efficiency analysis, suggesting that the efficiency would be enhanced if the GBs are doped to a considerable length-scale by diffusion of impurities in the emitter [9,10]. In this case, the minority carriers become majority carriers at the type-inversed GB, and such minority-carrier collections can suppress the negati
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